Photometry Lab 2020 PDF

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Lab on photometry...

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Photographic Technology Lab: Photometry Reading: Basic Materials & Processes of Photography Chapter 1 Name: Grade / 97

Objectives To gain experience in the application of a number of photometric concepts including: the inverse-square law, the effect of voltage variations on light output, the effect of age on the color temperature of a light bulb and the spectra of common light sources.

Required Handheld exposure meter (if you have one) Note: Throughout the lab, keep in mind to include units where necessary.

Part I: Candle Power 1. ( / 2) As a class we will measure the illuminance from a wax candle at a distance of 1 meter with a lux (metercandle) meter. Enter this measurement into Table 1. Table 1 Candle Power Illuminance

Candle 1.3 lux

2. ( /2 ) The value should have been 1 lux. Was it? No, the value was not 1 lux. 3. ( / 2) What may have caused the measured value to be something other than 1 lux? The illuminance of a candle varies depending on the amount of oxygen supplied to it.

Part II: Inverse Square Law The inverse-square law states that light from a point source varies inversely with the distance squared. In this section of the lab we will be taking measurements from several point sources and one broad area source to demonstrate this principle. 1.

( /3 ) As a class we will measure the illuminance of a wax candle from the distances listed in Table 1. Enter them into the table as the measurements are made.

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Photographic Technology Lab: Photometry

Table 1 Candle Illuminance at Increasing Distances Distance Illuminance 6 inches 40 lux 12 inches 10 lux 18 inches 6.6 lux 2.

( / 2) A candle can be considered a point source. Should it follow the inverse square law? Yes, it should. 3.

( / 4) Given the illuminance that you measured at 6 inches (1 unit of distance) and your understanding of the inverse square law, what should the illuminance have been at: Show your work: 40=i/12 Intensity= 40 cd/m2 a. 12 inches? E=40/22 E=10 lux b. 18 inches? E=40/32 E=4.44 lux

4.

( /2 ) How do the values you calculated in question 3 compare to the measured values in Table 1? The calculated illuminance at 12 inches is equal to the measured illuminance at 12 inches, but the calculated illuminance at 18 inches is lower than the measured illuminance at 18 inches.

5.

( /3 ) Measure the luminance at the distances provided below from a 75-watt bare lamp in an otherwise darkened room. Take a spot meter reading of the area inside the marked circles in units of cd/m2. Enter these measurements into the “Measured Luminance” column of Table 3.

Table 2 Bare Lamp Luminance Data Actual Unit Measured Distance Distance Luminance 3 inches 1 unit 1200 cd/m2 6 inches 2 units 420 cd/m2

Theoretical Luminance 1200 cd/m2 300 cd/m2

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Photographic Technology Lab: Photometry 9 inches 12 inches 15 inches 18 inches

3 units 4 units

150 cd/m2 80 cd/m2

133.33 cd/m2 75 cd/m2

5 units

34 cd/m2

48 cd/m2

6 units

24 cd/m2

33.33 cd/m2

6.

( / 5) Using the inverse square law it is possible to calculate theoretical luminance values for each distance interval. Fill out the “Theoretical Luminance” column of Table 2 starting with the luminance reading recorded at 3 inches from the bare lamp.

7.

( /5 ) Make an x-y scatter plot with smooth lines that shows the actual spotmetered measured luminance values at the 3-inch distance intervals compared to the theoretical luminance values. This plot should have the distances along the xaxis and luminance values from Table 3 as two separate series. Insert the plot here.

Measured Luminance vs. Theoretical Luminance 1400

Luminance (cd/m^2)

1200 1000 800

Measured Luminance Theoretical Luminance

600 400 200 0

2

4

6

8

10

12

14

16

18

20

Actual DIstance (Inches)

8.

( /3 ) Take spot meter readings of the luminance from the surface of a transparency illuminator (also known as a lightbox) at distances of 3 and 6 inches. Take these measurements from the center of the illuminator. Also, calculate the ratio of luminance’s. Enter these values into Table 4. Table 4 Luminance Readings from a Transparency Illuminator 1800 Luminance at 3 inches Luminance at 6 inches 1800

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Photographic Technology Lab: Photometry Ratio of luminance

1:1

Answer the following questions. ( /2 ) According to the inverse square law, the ratio between measurements at 3 and 6 inches from the bare lamp setup in step 5 should theoretically be: The ratio for the bare lamp setup should be 4:1 according to the inverse square law 9.

10. ( / 2) According to the inverse square law, the ratio between measurements at 3 and 6 inches from the transparency illuminator in step 8 should theoretically be: The ratio of the transparency illuminator should be 4:1 according to the inverse square law but is only 1:1 because it is not a point source.

Part III: Distribution of Light 1.

( / 3) Measure the illuminance with a lux meter at a distance of 30 cm from a 75watt reflector lamp, on-axis (0 degrees) and at 15 degree increments up to 80 degrees in one direction and -80 degrees in the other direction as demonstrated. Repeat this procedure with the 75-watt bare lamp. Record your values into Table 5 below. Table 5 Reflector and Bare Lamp Light Distribution Degree Reflector Lamp (Lux) Bare Lamp (Lux) 80 400 700 75 560 800 60 1300 840 45 2600 900 30 3200 900 15 4000 840 0 4200 900 -15 4000 1000 -30 3400 900 -45 1800 970 -60 640 900 -75 450 970 -80 340 900

2. ( /5 ) Plot the distribution of light for both light sources on the same graph. The x-axis of the graph should go from -80 to 80 degrees. Insert the plot here.

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Photographic Technology Lab: Photometry

Illuminance of Reflector Lamp vs. Bare Lamp 1200

Illuminance (Lux)

1000

800 Reflector Lamp (Lux) Bare Lamp (Lux)

600

400

200

0

0

2

4

6

8

10

12

14

16

Degrees

Part IV: New vs. Used Lamps ( / 7) Measure illuminance at a distance of 1 foot from the new and used 3200K photoflood lamps with a lux meter and record the readings. Measure the color temperature at the same distance with the color temperature meter. Calculate the Mired values and the Mired shift that has occurred due to the usage difference of the bulbs. Table 6 Color Temperatures of New and Old Lamps New Lamp Illuminance (in lux) 400 Color Temperature 3030 K Mired Value 330.03 Mired Shift (New – Used)

Used Lamp 320 2980 K 335.57 -5.54

1. ( / 2) Was there a color temperature difference between the two lamps. If so, was the used lamp warmer or cooler in color temperature? The new lamp was warmer than the used lamp by 5.54 Kelvin

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Photographic Technology Lab: Photometry Part V: Voltage Change ( /6 ) Measure the illuminance with a lux meter and the color temperature with a color temperature meter of a 75-watt lamp at a distance of 30cm at voltages of 120V, 110V and 100V. Record the data into Table 7 below. Table 7 Voltage changes and color temperature Voltage Illuminance (Lux) 120 volts 3000 110 volts 2400 100 volts 1600

Color Temperature (K) 2500 2430 2360

1. ( / 2) As voltage is decreased, what conclusion can be drawn between the relationship of voltage, color temperature and illuminance? Explain. As voltage decreases, both illuminance and color temperature decrease.

Part VI: Diversity of Light Sources 1. There are five different lights provided for you. The light source types and corresponding specifications are provided at the base of each bulb. 2. Turn each light source on one at a time by flipping the switches on the baseboard. 3. Measure the color temperature at a distance of 1 foot from each bulb. 4. ( 10/10 ) Calculate the delta (change) in color temperature between what you measured and the manufacturer’s published color temperature value. Table 8 Household Light Bulb Color Temperatures Light Source

Color Temperature (K)

Delta Color Temperature

Compact Fluorescent 550 Lumens 2700K

3040

340K

Halogen (40W) 455 Lumens 2850K

2690

160K

Tungsten (60W) 590 Lumens 2850K

2540

310K

LED 450 Lumens

3100

-100K

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Photographic Technology Lab: Photometry 3000K LED 490 Lumens 5000K

4910

90K

5. ( /5 ) Commenting on each light source separately, how did the published color temperature compare to the measurement that you made? Are the published values higher or lower? Comment on each if the measured difference, if any, indicates a warmer or cooler shift in color temperature. Remember that the lower the color temperature the warmer the light source. a.

Compact Fluorescent

The advertised color temperature is warmer than the actual bulb. b.

Halogen

The advertised color temperature is warmer than the actual bulb. c.

Tungsten

The advertised color temperature is warmer than the actual bulb. d.

LED 3000K

The advertised color temperature is colder than the actual bulb. e.

LED 5000K

The advertised color temperature is warmer than the actual bulb. An image will be provided on my courses that was captured as follows: a camera was connected to the LCD. All five light bulbs on at the baseboard were turned on. We placed a diffraction grating in front of the camera lens so that the emission spectrum of each bulb appears a few feet above the bulbs themselves and captured the image. Answer the following questions based on your observations. 6. ( /2 ) Out of the five light sources present there are four unique light types: compact fluorescent, halogen, tungsten, and LED. One light type does not produce a continuous spectrum of white light. Which bulb type does not have a continuous spectrum? The compact fluorescent bulb does not have a continuous spectrum. 7. ( /2 ) What visual evidence do you have to support this conclusion? There are spaces of black between the color while the other bulbs have continuous spectrums with no black disrupting the colors.

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Photographic Technology Lab: Photometry 8. ( /4 ) Based on your observations of each bulb’s spectra and your knowledge of the Color Rendering Index, a. which light source would have the highest CRI and why? The halogen and tungsten bulbs would have the highest CRI’s because they are both incandescent light sources and the colors near these bulbs appeared to be the closest to their actual values. b. which light source would have the lowest CRI and why? Both of the LED lights would have the lowest CRI because LED technology is relatively new and has not developed enough to provide accurate representations of colors and thus they have a lower CRI. 9. ( / 2) Two of the bulbs are LED: one with a color temperature of 3000K and one of 5000K. Is this difference evident from viewing the spectra? Explain how you came to this conclusion. Yes there is a difference between the two bulbs. The 5000K bulb has much more light blue/cyan visible in its spectrum and the spectrum itself is longer than the 3000K bulb. 10. ( / 5) In your estimation, which of the five bulbs has the least red light in its emission? Would this make the resulting quality of the light appear warmer or cooler? Because of its higher color temperature, the 5000k LED has the least red light in its emission, causing it to appear cooler. 11. ( / 5) It is nearly impossible to objectively say which light source is neutral by just looking at all of them turned on. What aspect of human vision accounts for this? Color constancy is the aspect of human vision that accounts for this issue.

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